Many short-term and long-term storage devices have bad sectors or otherwise unusable areas. MRAM (magnetic random access memory), flash memory, atomic resolution storage (ARS) devices, and NROM (nitride read-only memory), for example, often include such defects. Typical bulk storage media, such as magnetic and optical disks, for example, also often include defects, such as inconsistencies in magnetic or optical coatings or other surface anomalies. In all cases, such unusable areas make portions of such media or other storage devices unsuitable for data storage. Nevertheless, it is desirable to use such media or devices for storage even though they contain defects. The defect areas are generally relatively small compared to the total storage area, and such defects may develop or first be detected only after normal use of the media or device has begun.
MRAM is a non-volatile memory usable for short-term and long-term data storage. MRAM is generally considered to have lower power consumption than short-term memory such as DRAM, SRAM and flash memory, and generally can perform read and write operations much faster (by orders of magnitude) than conventional long-term storage devices such as hard drives. In addition, MRAM is more compact and consumes less power than hard drives. MRAM is also usable for embedded applications such as extremely fast processors and network appliances. Use of MRAM thus provides various advantages in certain situations and environments.
In the case of bulk storage media, traditional schemes for defects management have relied on using a portion of the media as a defect management area in order to present a media that, although including defective areas, appears as if it were defect-free. Accordingly, upon a manufacturer's formatting of the media for subsequent use in data storage, an analysis of the storage areas occurs and defective areas are marked as unusable. A list of the defective areas is stored on the media. A defect management system reads the list from the media and uses it to map defective areas to spare areas on the media. In order to provide media that includes a particular amount of available user storage area, logical addresses of the user data areas are “slipped” into the defect management area so as to omit the physical addresses of the defective areas and, thus, present seemingly defect-free logical media.
As defective areas may develop or be discovered during actual use of bulk storage media, there have been methods of providing redirection for or replacement of such defective areas to predefined available spare areas. Defective areas discovered after bulk storage media is formatted, for example, typically are called “grown” defects. Systems operate to remap or redirect the logical address associated with an area determined to be defective to the logical address of the predefined spare area. Therefore, a manufacturer's initial formatting of the media can include establishing predefined user data areas and corresponding predefined spare areas for redirection or remapping of grown defects. For example, with magneto-optical (MO) disks, a MO drive recognizes the particular media and uses both the predetermined user data areas and predetermined spare areas (defect redirection or remapping areas). Such spare areas have been interspersed with the user data areas throughout the media at various intervals, e.g. the first or last “n” sectors of a track, thus establishing zones within the media wherein an amount of user data area and its corresponding spare area are established. Defect management tables have been provided to allow the drive to properly read and write user data within these zones without encountering a defective area. Such management tables store a list of used spare areas and are used to determine spare areas available for remapping. Defective areas are mapped out by reassigning their logical addresses to the predefined spare areas.
With “linear replacement” defect mapping, a defective sector is simply mapped to a predefined spare sector. Access times for bulk media with linear replacement mapping however, can increase undesirably. To reduce the access latency of linear replacement mapping, “sector slipping” is also used. With sector slipping, the assignment of a logical sector number to a physical sector number skips over the defective sector, and all of the logical sector numbers are “slipped” by one. Sector slipping generally is not considered to lend itself well to use with grown defects, because of the large amount of remapping that must occur.
Spare areas typically are disposed at predetermined locations or intervals in the media. Such locations or intervals typically are chosen “blindly”, based on the physical makeup or structure of the storage device, i.e. regardless of their relative likelihood of failure. If a storage device is relatively free of defects, therefore, perfectly usable spare areas of the storage device may never be used and thus may be “wasted.” If certain sectors allocated for user data are of questionable quality, on the other hand, a large number of grown defects may arise, increasing access time and potentially causing other problems.